He now may face multiple challengers on the ballot this fall, creating a chance for a Green Party candidate to siphon votes from the left.

The Green Party earned access to Montana’s ballot after turning in thousands of signatures just before a critical deadline.

Montana Democrats are crying foul and want the Green Party booted from the ballot.

They suspect a GOP firm is behind the petition drive, and have filed a complaint with the Montana Commissioner of Political Practices alleging that a Nevada-based GOP firm violated state law by failing to report its spending on behalf of the Green Party.

The Montana Democratic Party says at least two of the signature gatherers identified themselves in social media postings as employees of Advanced Micro Targeting, a political consulting firm in Nevada that usually works with Republicans. Six of the 13 signature gatherers for the Green Party effort live in states other than Montana.

“The evidence suggests that the Republican-linked Nevada firm Advanced Micro Targeting, which has a history of shoddy and unethical petition-gathering practices, was behind this effort,” Nancy Keenan, the executive director of the Montana Democratic Party, said in a statement.

Billy Rogers, the owner of Advanced Micro Targeting, did not respond to an email seeking comment. Calls to the phone number listed on the company’s website went unanswered.

Democrats have also filed a challenge in court, alleging that Green Party backers did not turn in enough signatures to meet state requirements for qualifying.

Tester will face the winner of a contested GOP primary, to be held on June 5 between state Auditor Matt Rosendale, former district judge Russ Fagg, businessman Troy Downing and state Sen. Al Olszewski.

He would also face a Libertarian and, unless the party is knocked off the ballot, the winner of a two-person Green Party primary that some Democrats fear could pull votes from Tester and cost him the seat.

Republicans see a case of just desserts.

Tester, they say, has benefitted from third-party candidates pulling votes away from his GOP rivals. In both of Tester’s previous races, the Libertarian candidate has won a higher percentage of the vote than the margin by which Tester won.

In 2012, Tester allies went so far as to spend money to bolster the Libertarian. That year, a super PAC funded by labor and environmental groups backing Tester spent $500,000 on a television ad campaign calling the Libertarian candidate, Dan Cox, “the real conservative.” Cox took almost 32,000 votes that year. Tester beat then-Rep. Denny Rehberg (R) by a margin of just 18,000 votes.

“Nobody had ever heard of Dan Cox when he filed in 2012,” said Erik Iverson, the longtime Montana Republican strategist who ran Rehberg’s campaign that year. “For sure, the Republican Party here is all too willing to help out the Green Party in any way they can, much like Tester did for the Libertarian.”

Adding to the mystery is that one of the Green Party candidates, Timothy Adams, used to be on the payroll of the state Republican Party. That has fueled speculation that Republicans are trying the same tactic as Tester did six years ago, bolstering a spoiler candidate specifically to divide Tester’s support.

In a letter to Green Party members posted on the state party website, Adams acknowledged his work with the state GOP, his past affiliation with the Libertarian Party and previous contributions to Democratic candidates.

“I have supported efforts to allow more choice and less corporate influence in healthcare through the Republican party,” Adams wrote. He pledged to “promote the core values of the Green Part [sic] Platform through my candidacy for U.S. Senate and if elected, service in Congress.”

Adams will face gallery owner Steve Kelly in the Green Party primary.

Ballot access laws in Montana are particularly onerous for a new party. That party must submit at least 5,000 valid signatures, spread between 34 state legislative districts.

Those who want to see the Green Party on the ballot in Montana turned in more than 10,000 signatures in the days leading up to a state deadline last month. Of those, about 9,500 were gathered by 13 people and filed on the final day on which a party can petition for ballot access, according to public records.

In a statement, state Green Party spokeswoman Danielle Breck said her party did not know if any outside group collected signatures on its behalf.

“We are unaware of any paid petitioning efforts taking place on our behalf,” Breck said in the statement. “We are a grassroots movement that, upon initiating our petition efforts more than a year ago, made a public appeal to our supporters asking for assistance gathering signatures.”

Iverson said it would not be surprising if some of the signatures gathered on behalf of the Green Party came from Republican voters.

“I don’t think any Republican in the state of Montana, in any shape or form, is saying this isn’t a good thing” for the GOP, he said. “Because it definitely is.”

Asked whether Tester’s campaign was worried that the Green Party nominee would siphon votes away from the incumbent, spokesman Chris Meagher said in an email: “No, we’re not worried about it.”

But that has not stopped the state Democratic Party from seeking to kick the Greens off the ballot. The state party said closer inspection of some of the signatures submitted on behalf of the Green Party would show many were not valid, and that the party had only turned in a sufficient number of signatures in 30 of the 34 legislative districts required by state law.

“While county clerks did an outstanding job with the little time they were given to review the signatures submitted to them, we believe a closer review shows the threshold required by law was not met,” Keenan said in a statement.

Iverson said Democrats who back greater ballot access were being hypocritical in challenging the Green Party’s nominating papers.

“You have Democrats trying to block access to the ballot for the Green Party. My guess is if the Republicans would try to do this for the Libertarians, it’d be front-page news out here,” he said.